RESPONSIVENESS IN VERTEBRATES 



temperature receptors, one specifically responsive to heat, the other to cold. 

 These receptors are widely distributed but, again, certain areas of the skin 

 contain more of one kind than of the other. For example, the lips are more 

 sensitive to heat than the hands are. Pain receptors are free nerve endings 

 in the skin of the entire body. A wide variety of stimuli result in impulses 

 interpreted by the brain as pain. Local anesthetics, narcotizing the pain 

 receptors, can abolish the sense of pain but leave intact the responses to 

 touch and temperature. 



Finally, other receptors are located in deep structures of the body, such as 

 muscles, tendons, joints, and visceral organs. Some of these provide informa- 

 tion that is useful in maintaining body posture and in equilibration. Others 

 are important for the maintenance of the normal internal environment of the 

 body by the autonomic nerves; this will be discussed in a later section. 



fig. 4.20. Surface of human tongue, showing 

 areas of localization of taste buds receptive 

 to each of four basic tastes. 



Reception, or stimulation of a sensory area, is followed by the establishment 

 of nerve impulses which are then conducted along the processes of the afferent 

 neurons associated with the area. The stimulus must reach a certain intensity 

 before the nerve endings respond. Increased stimulation will increase the 

 frequency with which nerve impulses are initiated in a particular neuron and, 

 also, increase the number of neurons in which impulses start. An increase in 

 intensity of the stimulus above the threshold value does not change the quality 

 of these nerve impulses which are of the all-or-none variety; if the impulse 

 fires at all, it fires at maximum intensity. 



The functions of the nervous system require a great deal of chemical energy 

 and exhibit measurable electrical phenomena. When man is in a resting 

 state, about 20 per cent of the oxygen entering the blood stream is utilized 

 in the brain. Less is known about the intricate and very rapid chemical 

 changes than about the closely correlated electrical changes that occur in the 



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